WIP: drm/msm: add rd logging debugfs

To ease debugging, add debugfs file which can be cat/tail'd to log
submits, along with fence #.  If GPU hangs, you can look at 'gpu'
debugfs file to find last completed fence and current register state,
and compare with logged rd file to narrow down the DRAW_INDX which
triggered the GPU hang.

Conflicts:
	drivers/gpu/drm/msm/msm_gpu.c

Conflicts:
	drivers/gpu/drm/msm/Makefile
	drivers/gpu/drm/msm/msm_drv.h